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Variability in the South Atlantic Anticyclone and the Atlantic Niño Mode

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  • 1 National Oceanic and Atmospheric Administration/Pacific Marine Environmental Laboratory, Seattle, Washington
  • 2 Department of Geology and Geophysics, Yale University, New Haven, Connecticut
  • 3 Department of Oceanography, University of Cape Town, Cape Town, South Africa
  • 4 National Oceanic and Atmospheric Administration/Pacific Marine Environmental Laboratory, Seattle, Washington
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Abstract

Previous studies have argued that the strength of the South Atlantic subtropical high pressure system, referred to as the South Atlantic anticyclone (SAA), modulates sea surface temperature (SST) anomalies in the eastern equatorial Atlantic. Using ocean and atmosphere reanalysis products, it is shown here that the strength of the SAA from February to May impacts the timing of the cold tongue onset and the intensity of its development in the eastern equatorial Atlantic via anomalous tropical wind power. This modulation in the timing and amplitude of seasonal cold tongue development manifests itself via SST anomalies peaking between June and August. The timing and impact of this connection is not completely symmetric for warm and cold events. For cold events, an anomalously strong SAA in February and March leads to positive wind power anomalies from February to June resulting in an early cold tongue onset and subsequent cold SST anomalies in June and July. For warm events, the anomalously weak SAA persists until May, generating negative wind power anomalies that lead to a late cold tongue onset as well as a suppression of the cold tongue development and associated warm SST anomalies. Mechanisms by which SAA-induced wind power variations south of the equator influence eastern equatorial Atlantic SST are discussed, including ocean adjustment via Rossby and Kelvin wave propagation, meridional advection, and local intraseasonal wind variations.

Pacific Marine Environmental Laboratory Publication Number 4140.

Current affiliation: GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany.

Corresponding author address: Joke F. Lübbecke, GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany. E-mail: jluebbecke@geomar.de

Abstract

Previous studies have argued that the strength of the South Atlantic subtropical high pressure system, referred to as the South Atlantic anticyclone (SAA), modulates sea surface temperature (SST) anomalies in the eastern equatorial Atlantic. Using ocean and atmosphere reanalysis products, it is shown here that the strength of the SAA from February to May impacts the timing of the cold tongue onset and the intensity of its development in the eastern equatorial Atlantic via anomalous tropical wind power. This modulation in the timing and amplitude of seasonal cold tongue development manifests itself via SST anomalies peaking between June and August. The timing and impact of this connection is not completely symmetric for warm and cold events. For cold events, an anomalously strong SAA in February and March leads to positive wind power anomalies from February to June resulting in an early cold tongue onset and subsequent cold SST anomalies in June and July. For warm events, the anomalously weak SAA persists until May, generating negative wind power anomalies that lead to a late cold tongue onset as well as a suppression of the cold tongue development and associated warm SST anomalies. Mechanisms by which SAA-induced wind power variations south of the equator influence eastern equatorial Atlantic SST are discussed, including ocean adjustment via Rossby and Kelvin wave propagation, meridional advection, and local intraseasonal wind variations.

Pacific Marine Environmental Laboratory Publication Number 4140.

Current affiliation: GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany.

Corresponding author address: Joke F. Lübbecke, GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany. E-mail: jluebbecke@geomar.de
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